Origin of Cryptography, Caesar Cipher

10 March 2019

Caesar Cipher is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the text is replaced by a letter some fixed number of positions down the alphabet.

You can watch this Khan Academy Video and read this article for more information.

Lets write encrypt and decrypt functions for Caesar Cipher Encryption.

Encryption Example:

input         : "hello", 5
output        : "mjqqt"

Decryption Example:

input         : "Oazsdmfgxmfuaze", 12
output        : "Congratulations"

We read every character one by one using for loop in Python. Instead of using alphabet string I used Python’s two build-in functions for shifting. ord() function converts character to its ascii code and the chr() function works vice-versa. Firstly we get the ascii number of the character, then add that shift value.

However, English alphabet starts from a(97) and ends at z(122) or for capitalize alphabet it’s A(65) to Z(90). In order to avoid other ascii characters first we need to remove characters before ‘a’, then shift it. Be careful here, because after ‘z’ again starts different characters, so we can prevent it to exceed 26 using modulo operator. Finally we add removed characters back and convert back from ascii to the char.

def encrypt(text, shift):
    cipher = ""
    for char in text:
        if char == ' ': # If it's space character add without any change
            cipher = cipher + char
        elif char.isupper():
            cipher = cipher + chr((ord(char) + shift - 65) % 26 + 65) # 65 for 'A'
        else:
            cipher = cipher + chr((ord(char) + shift - 97) % 26 + 97) # 97 for 'a'
    return cipher

def decrypt(text, shift):
    cipher = ""
    for char in text:
        if char == ' ':
            cipher = cipher + char # If it's space character add without any change
        elif char.isupper():
            cipher = cipher + chr((ord(char) - shift - 65) % 26 + 65) # 65 for 'A'
        else:
            cipher = cipher + chr((ord(char) - shift - 97) % 26 + 97) # 97 for 'a'
    return cipher

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Here is another way to write function using alphabet string.

def encrypt(text, shift):
    lower_case_alphabet = "abcdefghijklmnopqrstuvwxyz"
    upper_case_alphabet = lower_case_alphabet.upper() # Capitalize
    cipher = ""
    for char in text:
        if char == ' ':
            cipher = cipher + char
        elif char.isupper():
            # Find index of char in upper_case_alphabet and shift it.
            cipher = cipher + upper_case_alphabet[(upper_case_alphabet.index(char) + shift) % 26]
        else:
            # Find index of char in lower_case_alphabet and shift it.
            cipher = cipher + lower_case_alphabet[(lower_case_alphabet.index(char) + shift) % 26]
    return cipher

def encrypt(text, shift):
    lower_case_alphabet = "abcdefghijklmnopqrstuvwxyz"
    upper_case_alphabet = lower_case_alphabet.upper()
    cipher = ""
    for char in text:
        if char == ' ':
            cipher = cipher + char
        elif char.isupper():
            # Find index of char in upper_case_alphabet and shift it.
            cipher = cipher + upper_case_alphabet[(upper_case_alphabet.index(char) - shift) % 26]
        else:
            # Find index of char in lower_case_alphabet and shift it.
            cipher = cipher + lower_case_alphabet[(lower_case_alphabet.index(char) - shift) % 26]
    return cipher